and see text Helcionelloida is an extinct group of ancient molluscs (phylum Mollusca).

These are the oldest known conchiferan molluscs, that is, they had a mineralised shell.

The majority of species were small (shells being about two millimeters in length and or diameter).

The shells of some genera, particularly Yochelcionella, also possessed a "snorkel"-like opening which was most likely used for breathing.

The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005[2] categorizes members of this taxon within the Paleozoic molluscs of uncertain systematic position, but that taxonomy to not use the name Helcionelloida.